A power supply unit is installed in a vehicle and provides an electric power to a motor to drive the vehicle. The power supply unit accelerates the vehicle by the motor using a discharge of the battery. The power supply unit is charged through regenerative braking of decelerating the vehicle or driving of the generator by an engine. In the power supply unit as a type of a stationary storage battery, the battery is charged through a charging power source, for example, a midnight power from a commercial power supply, a solar cell, or the like. In the battery used in such a manner, It is important to reduce a degradation of the battery or use the battery safely. When the battery is over-discharged or over-charged, the degradation proceeds, and it decreased a cycle life. This disadvantage is resolved by precisely detecting the remaining capacity (state-of-charge [SOC]) of the battery which varies by charging and discharging, and limiting an available power in charging and discharging of the battery. The remaining capacity of the battery is computed through an integrated value by integrating a current value in charging and discharging of the battery. In this method, the remaining capacity [SOCi (%)] is computed through subtracting the integrated value of a discharging current, adding the integrated value of a charging current The integrated SOCi is shown, for example, 100% in the full charge, as the ratio (%) of a remaining capacity (Ah) to the full charge capacity (Ah). The current integrated remaining capacity SOCi doesn't necessarily always show the accurate remaining capacity of the battery. An error in the current integrated remaining capacity happens, and it is caused by a current measurement error, the battery temperature and the like
In order to resolve this disadvantage, the computed method of the real remaining capacity [SOC (%)] based on both the current integrated remaining capacity [SOCi (%)] and the remaining capacity [SOCv (%)] detected through the voltage is developed in the charging or discharging stage (see Patent Literature 1).
In the remaining detection method, the remaining capacity SOCi is computed through an integrated value by integrating a current value in charging and discharging of the battery, in addition, the remaining capacity [SOCv (%)] detected through the open circuit terminal voltage of the battery, so the real remaining capacity [SOC (%)] is computed based on both the current integrated remaining capacity [SOCi (%)] and the remaining capacity [SOCv (%)]. The remaining capacity [SOCv (%)] is computed in the following equation.
(1) By detecting the current and the voltage of battery, the open circuit voltage Voc is computed. In order to detect the open circuit voltage Voc through the current and the voltage of battery, detecting the characteristics between the current and the voltage, and an internal resistance R is computed through the inclination in the characteristics between the current and the voltage, so the open circuit terminal voltage Voc is calculated in the following equation.Open circuit terminal voltage Voc=Detected voltage (V)+Discharging current (I)×Internal resistance (R).(2) From the calculated open circuit terminal voltage, the remaining capacity SOCv through the memorized characteristics between the open circuit terminal voltage and the remaining capacity is computed. In order to detect the remaining capacity through the open circuit terminal voltage, the remaining capacity corresponding to the open circuit terminal voltage Voc is stored in a memory as the characteristics between the open circuit terminal voltage and the remaining capacity.